Project Summary The integrity of intestinal mucosa is vital for nutrition absorption and defense against pathogens. Disruption of intestinal epithelial homeostasis leads to a variety of intestinal disorders, such as chronic inflammation, mucosal ulceration, and cancer. Claudins are the major tight junction (TJ) integral membrane proteins with at least 24 members in mammals. Although more than 10 claudin family members are expressed in human and mouse intestines, only claudin-7 and -15 are known to be essential since their deletion causes severe intestinal phenotypes. However, little is known about the mechanisms by which they function in epithelial injury, repair, and regeneration in health and diseases. Our laboratory has generated the first claudin-7 knockout mouse model. Claudin-7 deletion in intestines leads to severe mucosal ulcerations, epithelial cell sloughing, and inflammation. Besides its conventional association with TJ, claudin-7 also interacts with integrin ?2 in the epithelial basolateral compartment of intestines. Deletion of claudin-7 reduces integrin ?2 expression, upregulates MMPs and cytokines, and evokes inflammatory responses. In addition, the balance between the intestinal epithelial stem cell (IESC) proliferation and differentiation is disrupted in Cldn7-/- intestines. These findings have led to the hypothesis that claudin-7 has a novel, previously unidentified non-TJ function in maintaining epithelial cell-matrix interactions and intestinal homeostasis through modulating inflammatory and stem cell signaling. The goal of this project is to investigate how claudin-7 serves as a key element required for intestinal functions. We propose two aims. Aim 1 will determine whether claudin-7 deletion induces intestinal inflammation through disruption of the TJ barrier function or cell-matrix adhesion mediated by MMPs. This aim will be achieved by determining (1) whether the TJ barrier function is disrupted with claudin-7 deletion associated with inflammatory responses, and (2) whether the increase of MMPs promotes or the inhibition of MMPs attenuates the inflammatory response in cultures and in animals. Aim 2 will determine if claudin-7 deletion induces intestinal inflammation, leading to the disruption of IESC proliferation and differentiation. Based on the gene and protein expression profiling showing the striking down-regulation of IESC marker Olfm-4, this aim will determine (1) the role of Olfm4 in inflammatory response upon LPS or TNF? treatment in intestinal epithelial cells with or without claudin-7 expression, and (2) whether the inflammatory signaling increases the stem cell proliferation and reduces the intestinal epithelial cell differentiation. This project will shed light on if claudin-7 deletion induces inflammation through disruption of the canonical TJ barrier or cell-matrix adhesion and how claudin-7 elicits inflammation to affect the fate of IESCs. Overall, the broad impact of this project will be identifying ?claudin-7-MMPs-IESCs functions? as potential targets through which claudin-7 controls intestinal homeostasis against inflammation.